The difference between 60% credence and 80% credence seems much smaller to me than the difference between 90% and 99%. Is there a reason there's no option between 90% and 99%? In your testing, have you found any well-calibrated users who answer 99% a non-trivial fraction of the time?

Yes, but it also depends on your knowledge base. Unfortunately right now we don't have a lot of questions, but I hope people will add more. Then, we hope, you'll have more questions for which you can answer 99%.